Different approaches have been explored to overcome the problem of volume expansion and contraction upon lithiation and delithiation of Si anodes. One approach is to use nanostructures, such as nanocrystals, nanowires, nanotubes or nanorods...Unfortunately, it is very difficult to produce these materials at a reasonable cost in the bulk quantities required for pragmatic applications. Additionally, decreasing the size and size dispersity of Si cannot alone effectively suppress specific volume changes or reduce particle-particle agglomeration.

An alternative approach to improve the stability of Si is by either forming an alloy with other ductile materials to act as a volumetric expansion buffer, or by using nanosized materials dispersed uniformly in a buffer matrix. The incorporation of buffer materials is advantageous as they reduce volumetric expansion and fracture during cycling. Typically, carbon materials have been used as the buffer material in different architectural configurations...However, in all of the above approaches, it is note well understood how ion and electron charge transfer kinetics are affected by addition of buffering agents with Si nanostructures, and whether the large irreversible capacity loss is caused by the Si material itself or by the buffering matrix holding the active material together.

...As the electrochemical lithiation/delithiation of crystalline Si leads to amorphorization in only a few cycles, some studies have explored the use of amorphous silicon (a-Si) for lithium ion battery applications, as there is no advantage to using crystalline Si. a-Si has other potential advantages over crystalline Si, including a smaller predicted volume expansion, shorter lithium ion diffusion lengths, and smaller charge transfer resistance. Nano-sized a-Si should offer even better tolerance to volume expansion/contraction processes.

—Murugesan et al.

The researchers prepared their electrodes by creating slurries of a-Si:H particles or Cu-coated a-Si:H as the active material, Super P carbon black as an electronic conductor, and PVDF dissolved in NMP as binder in a 70:20:10 ratio by weight. The 100% a-Si and carbon electrodes were made with 90:10 ratios of active materials and PBVDF dissolved in NMP as binder, respectively.

The resulting electrodes were used in coin cells (2032 type) using metallic lithium as the counter electrode and 1M LiPF6 dissolved in ethylene carbonate (EC) and diethyl carbonate (DEC) in a 1:1 volume ratio as the electrolyte. The team tested a-Si:H particles of varying sizes for electrochemical lithium insertion.

They found that the smallest a-Si:H particles tested (380 nm diameter) showed a relatively high capacity of 580 mAh g^-1 in the first cycle that dropped significantly to 165 mAh g^-1 in the second and then further to 40 mAh g^-1 in subsequent cycles at a current rate of 100 mA g^-1. Capacities of particles of different sizes differed by 10 to 20 mAh g^-1 with no apparent size dependence. Average capacity was limited to about 50 mAh g^-1. This storage capacity is quite low compared to the maximum storage capacity of 3,579 mAh g_-1 of crystalline silicon, the team noted.

By contrast, the copper-coated a-Si:H particles showed a specific charge storage capacity of 600 mAh g^-1 at 100 mA g^-1 load—nearly 7 times higher than that of the pristine a-Si:H particles and higher than that of the theoretical capacity of graphite anodes (372 mAh g^-1). The Cu-coated particles did not lose capacity and degrade with successive cycling; rather, they showed an increase in charge storage capacity with an increase in number of cycles.

Cu coated a-Si:H particles exhibited significantly enhanced lithium storage capacity over pristine a-Si:H particles of about 7 fold. The presence of Cu helps to suppress the solvent decomposition and enhance the lithiation/delithiation process taking place in a-Si:H particles and the role of the Cu layer in these processes. This chemical approach of coating Cu over a-Si:H particles shows great potential towards developing advanced anode materials for lithium ion batteries.

—Murugesan et al.

Resources

• Sankaran Murugesan, Justin T. Harris, Brian A. Korgel, and Keith J. Stevenson (2012) Copper-coated Amorphous Silicon Particles as an Anode Material for Lithium-Ion Batteries. Chemistry of Materials doi: 10.1021/cm2037475